The invention relates to a photographic copying apparatus and to a process for operating such a copying apparatus.
Photographic copying apparatuses carry out further processing on photographic film that has been developed in a film-developing machine. The photographic film, which generally is not notched, passes through several treatment stations in a so-called film path, for example a measuring station, an exposure station or, finally, a cutting station. In the measuring station, the data required to determine the correct exposure are detected, as is also the position of the data on the film, and the data are fed to a calculating and evaluation station. Then, on the basis of the calculated exposure data, for example for the primary colours red, green and blue, and on the basis of the associated position data on the photographic film, the exposures are carried out in the exposure station. For that purpose, the measurement data obtained in the measuring station must be unambiguously associated with a site on the film and, in the repositioning station, in this case the exposure station, which is separated spatially from the measuring station, that site must be positioned with a very small tolerance. For example, a master on the film (that is to say, for example, a negative image) must be arranged in the region of an exposure window with an acceptable deviation of no more than 0.5 mm. It is obvious that that is no easy undertaking in modern photographic copying apparatuses having high throughput capacities, especially in so-called heavy-duty printers having a processing capacity of 20,000 or more images per hour.
For example, great difficulties arise as a result of tolerances and wear effects of the mechanics of the copying apparatus. The film is generally transported through the film path by friction between the film surface and the transport rollers. Thus, for example, the radius of the film transport rollers may deviate from the nominal radius, both as a result of manufacturing tolerances and as a result of wear caused by abrasion. The roller radius may also vary as a result of the temperature-induced "flexing" of the transport rollers which are provided with a layer of rubber. A further substantial problem, which occurs as early as in the measuring station and which can make itself felt in a detrimental manner also in the further treatment station (for example the exposure station or the cutting station), which is separated spatially from the measuring station, is the slip between the film surface and the transport rollers. Although it is possible to reduce the slip problem by using transport rollers having a very high coefficient of friction, such rollers generally exhibit a very high degree of abrasion which, in turn, see above, changes the roller diameter. Abrasion-resistant transport rollers having smooth surfaces, on the other hand, lead to increased slip. Finally, an increase in the contact pressure of the transport rollers on the film is also only possible to a slight degree because there is a risk that the surface of the film will be damaged.
DE-A-3 833 731 relates to a process for processing developed photographic films and to an apparatus for carrying out that process, which process is intended to eliminate the problems described. In the measuring station, the position of the image fields is ascertained by photoelectric scanning in a measuring slot extending perpendicular to the direction of transport. In a processing station that is displaced with respect thereto by a relatively great film length, the film is processed in spatial association with the image fields ascertained. The transport of the image field from the measuring station to the processing station is controlled on the basis of a measurement of the transported film length. For that purpose, image edges recognised in the measuring station and/or marked changes in density in the region of the image fields are stored with their values and longitudinal co-ordinates. Shortly before reaching the processing station, the film is again scanned for image edges and/or marked changes in density and the associated longitudinal co-ordinates are ascertained. The two scans are compared with one another and, if there are variations in the longitudinal co-ordinates, the path length measuring devices for the second scan are corrected accordingly.
In the case of that process and the apparatus of the prior art, the accuracy of film positioning is based essentially on the accuracy of the detection of an image edge and/or of a marked shift in density in an image field and on the accuracy of the determination of the associated longitudinal co-ordinate on the film. It is known, however, that, when detecting the image edges and/or the marked changes in density within an image field and when recording the associated longitudinal co-ordinates, errors may occur which are on the one hand system-induced and, on the other, are of a statistical nature. In addition to the afore-mentioned deviations of the roller radii of the transport rollers from their nominal value and the temperature-induced "flexing" of the rollers, system-induced errors can occur, for example, as a result of mechanical tolerances of the density-change sensors. The sensors for detecting the longitudinal co-ordinates of the film, usually a counting mechanism for the steps of the motors for the transport rollers, can also contribute to deviations in that qualifying errors on the part of the counting mechanism occur or in that the control electronics do not supply absolutely exact control pulses. Added to those system-immanent errors are, however, also the so-called statistical sources of error. Apart from the afore-mentioned slip, for example the positioning accuracy of the stepping motors for the transport rollers may vary. In many cases, the sensors also have a statistical noise which can distort the detected events or the recorded longitudinal coordinates of the film. All those possible sources of error contribute to the fact that the film cannot always be positioned with the desired accuracy because the apparatus and the process of the prior art are geared to the recording and the recognition of a single event. Furthermore, that known process requires the stepping motors for the film transport rollers in the measuring station and in the repositioning station to be driven at the same rate, but, in many cases, it is desirable for the stepping motors in the repositioning processing station to be driven with a greater step size and at a correspondingly reduced step frequency.